1. Field of the Invention
The invention relates to a protective coating and to a component on which the coating is applied, particularly on a gas turbine component or another component made from a nickel-based or cobalt-based alloy.
Numerous compositions of protective coatings of alloys which primarily contain nickel, chromium, cobalt, aluminum and a reactive element of the rare earths have been developed and tested. One such coating has become known heretofore from U.S. Pat. No. 4,005,989, for example. Other coatings of this general type are known from U.S. Pat. No. 3,928,026 to Hecht et al. and from an article entitled "The Durability and Performance of Coatings in Gas Turbine and Diesel Engines" by J. Fairbanks and R. Hecht, Materials Science and Engineering, 88 (1987), pages 321-330; these papers discuss the ductility properties of such coatings and their significance in the gas turbine environment. From U.S. Pat. No. 4,034,142, it is also known that an additional constituent, silicon, can further improve the properties of such protective coatings. Although the relatively wide ranges of the various elements in these documents, in fact, do suggest qualitatively a way to create protective coatings resistant to high-temperature corrosion, the compositions disclosed are not sufficiently specific quantitatively for all purposes.
German Patent 23 55 674 discloses further compositions for protective coatings, but they are not suitable for uses or applications of the type which can occur with stationary gas turbines having a high inlet temperature.
The ductility of protective coatings for gas turbine components is further discussed in an article by W. Schmidt and G. Lehnert, in Zeitschrift fur Werkstofftechnik (magazine for materials science), 15 (1984), pages 73-82. The term ductile brittle transition temperature (DBTT) is introduced in that paper, and thus defined for the purpose of this text, for a combination of a substrate and a protective coating. In this context, the protective coating is deemed to be brittle if it exhibits stress cracking at stresses below the 0.2% elasticity limit of the substrate. The protective coating is deemed to be ductile if it exhibits stress cracking only above the 0.2% elasticity limit of the substrate. The ductile brittle transition temperature of the combination is defined to be the threshold temperature below which the protective coating is brittle and above which it is ductile. In gas turbine applications it is most desireable to provide protective coatings in which the ductile brittle transition temperature (DBTT) is fairly low when combined with suitable substrate materials for the components to which the coatings are to be applied.
It is accordingly an object of the invention to provide a protective coating and a component in which the coating has high resistance to corrosion both at medium temperatures and at high temperatures, and in which the combination of the component and the protective coating has a low ductile brittle transition temperature. The corrosion properties should be improved in the temperature range from 600.degree. to approximately 1150.degree. C. so that such protective coatings can be used particularly in stationary gas turbine systems which operate at partial load or full load.
With the foregoing and other objects in view, there is provided in accordance with the invention, a protective coating resistant to corrosion at medium and high temperatures on a component formed of nickel-based or cobalt-based alloy, essentially consisting of the following elements (in percent by weight): 25 to 40% nickel, 28 to 32% chromium, 7 to 9% aluminum, 1 to 2% silicon, 0.3 to 1% of at least one reactive element of the rare earths, at least 5% cobalt; and impurities, as well as selectively from 0 to 15% of at least one of the elements of the group consisting of rhenium, platinum, palladium, zirconium, manganese, tungsten, titanium, molybdenum, niobium, iron, hafnium, and tantalum, the combination of the component and the protective coating having a ductile brittle transition temperature below 600.degree. C., and preferably below 500.degree. C. Preferably, the chromium content is 29 to 31%.
In this regard, the selective inclusion of a particular element of the last-mentioned group of elements is based upon knowledge that the element does not worsen the properties of protective coatings but, instead, actually improves them, at least under certain circumstances.
In accordance with an added feature of the invention, the ductile brittle transition temperature of the combination component/coating is lower than approximately 450.degree. C. and, advantageously, lower than approximately 400.degree. C.
In accordance with another feature of the invention, the protective coating is applied to a component which is a nickel-based superalloy consisting essentially of the following elements (in percent by weight): 0.08 to 0.1% carbon, 12 to 16% chromium, 8 to 10% cobalt, 1.5 to 2% molybdenum, 2.5 to 4% tungsten, 1.5 to 4.5% tantalum, 0 to 1% titanium, 0 to 0.1% zirconium, 0 to 1% hafnium, a minor addition of boron, and a balance of nickel.
The following properties or significance can be ascribed to the various constituents of the protective coating:
Cobalt, as a constituent, effects good corrosion properties at high temperatures.
Nickel improves the ductility of the coating and reduces interdiffusion with respect to the nickel-based base materials. The preferred range for nickel is from 25 to 35% and preferably approximately 30%.
Chromium improves the corrosion properties at medium temperatures up to approximately 900.degree. C. and promotes the formation of an aluminum oxide covering film. The preferred range for chromium is from 28 to 32% and in particular approximately 30%.
Aluminum improves the corrosion properties at high temperatures up to approximately 1150.degree. C. The share of aluminum should be about 7 to 9%, the preferred share being from 7.5 to 8.5% and, in particular, approximately 8%.
Silicon reinforces the action of chromium and aluminum and promotes the adhesion of the protective oxide film. A favorable range for the silicon constituent is 1 to 2%, preferably approximately 1.5%. By means of the addition of silicon, the share of aluminum and/or of chromium can be reduced from the high content actually desired for good corrosion properties to the more favorable values for ductility, without thereby impairing the corrosion properties.
The action of a reactive element, in particular yttrium, is known per se. The preferred range thereof is from 0.3 to 1% and, in particular, approximately 0.6%.
In the preferential ranges given, tests have shown particularly good corrosion properties for the protective coatings for applications in gas turbines having an inlet temperature above 1200.degree. C.
From prior art literature, various elements have become known which do not impair the properties of a protective coating, but rather, in some aspects actually improve them when admixed in a range less than 15%, and in particular in a share of only a few percent. The invention of the instant application is also intended to encompass alloys with such admixtures.
An element which has scarcely been given any consideration for protective coatings, namely rhenium, can markedly improve the corrosion properties if it is admixed in a proportion of from 1 to 15%, preferably 4 to 10%, and in particular approximately 7%.
Although rhenium is not as expensive as most noble metals, as a constituent of a protective coating it can produce properties just as good as those achieved, for example, by platinum, and can also be effective even when it constitutes only a small share of the protective coating.
The coatings according to the invention are applicable by plasma spraying or vapor deposition (PVD), and they are particularly well suited for gas turbine blades formed from a nickel-based or cobalt-based superalloy. Other gas-turbine components, as well, particularly in gas turbines having a high inlet temperature of above 1200.degree. C., for example, may be provided with such protective coatings. The special composition of the coating according to the invention has proved in tests to be a particularly suitable selection for stationary gas turbines having a high inlet temperature. Such tests will be discussed in the following.